An oat-legume-grass mixture can be seeded successfully in one trip over the field with a single-hopper machine. Seed separation isn't a problem, and you can achieve uniform seed distribution and a satisfactory planting depth.
Casselman, T. W. and Scholl, J. M.
"You Can Simplify Oat-Legume-Grass Seeding,"
Iowa Farm Science: Vol. 15
, Article 5.
Available at: http://lib.dr.iastate.edu/farmscience/vol15/iss7/5